This invention relates to a method for preparing catalysts useful in the production of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly it is directed to the preparation of catalysts suitable for producing maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3-butadiene or a mixture thereof.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3-butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. A conventional method of preparing these catalysts involves dissolving a vanadium compound, a phosphorus compound, and if desired, promoter element compounds in a reducing medium under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered by evaporating down the precursor-containing medium, or causing precipitation of the precursor, followed by calcining to provide active catalytic material.
THe use of gaseous HCl as a solvating and reducing agent for vanadium is disclosed in U.S. Pat. No. 4,002,650 where the vanadium and phosphorus compounds are dissolved in an aqueous solution.
European Patent Appln. No. 3,431 discloses the preparation of vanadium phosphorus mixed oxide catalysts by the dissolving and heating of a vanadium compound in an aqueous concentrated HCl solution, with subsequent addition of phosphoric acid. The solution is evaporated down to dryness and the resulting precursor solid is recontacted with water with boiling to remove inactive phase material, while the precursor solid remains suspended in the water. The solid is filtered out of the suspension, dried and calcined.
The use of gaseous HCl as a solvating and reducing agent for vanadium is also described in U.S. Pat. No. 4,043,943 where the vanadium and phosphorus components are present in solution in liquid organic medium.
The catalyst precursor is recovered by evaporating or distilling down the organic medium, or, by effecting precipitation by adding inferior organic solvents for the precursor or by forming a supersaturated solution. This patent teaches that only a minor amount of water is acceptable in the medium at the time of precipitation (including evaporation or distillation).
It has been found that superior vanadium-phosphorus mixed oxide catalysts may be prepared in organic media. However, the conventional methods for recovering the catalyst precursor from organic solution, particularly by evaporation of the organic medium, poses difficulties in the commercial scale-up of catalyst production. For example, the liquid media which must be evaporated off in large quantities form flammable vapors.